High Pressure Synthesis of a New 0201 type Oxychloride High Tc Superconductor by ``Apical Oxygen Doping''

We present our recent research works on high pressure synthesis of high Tc superconductors in the alkaline earth copper oxychloride system of Sr$_{2}$CuO$_{2}$Cl$_{2}$, the isostructure compound with La$_{2}$CuO$_{4,}$ where the apical site relative to the [CuO2] plane is chlorine rather than the usual oxygen. Using the ``apical oxygen doping'' mechanism, i.e., partially substituting the oxygen for chlorine, we succeeded in synthesizing a new 0201 type superconductor Sr$_{2}$CuO$_{2+\delta }$Cl$_{2-x}$. Nearly single phase of Sr$_{2}$CuO$_{2+\delta }$Cl$_{2-x}$ superconductors were obtained, showing superconducting transition at 35 K for the as-prepared sample.